There has been a demand for further size and weight reductions of components to be incorporated in electronic equipment such as mobile equipment. For example, a filter used in mobile equipment is requested to be reduced in size and also to be variable in central frequency and frequency band. As one type of filters that meet these requirements, a filter using an acoustic thin film resonator has been known (see Patent Documents 1 to 2).
First, with reference to FIG. 17, the following describes a conventional acoustic thin film resonator described in Patent Document 1.
FIG. 17 shows a cross-sectional structure of an acoustic thin film resonator. This acoustic thin film resonator has a structure in which a piezoelectric body 2 and semiconductor layers 1 and 3 that are provided respectively on both surfaces of the piezoelectric body 2 are sandwiched between an upper electrode E2 and a lower electrode E1. This structure is supported on a substrate S via an acoustic mirror layer AS. The acoustic mirror layer AS is formed of a combination of materials that have a large difference in acoustic impedance. In this acoustic thin film resonator, an electric field is applied in a thickness direction by the upper electrode E2 and the lower electrode E1, and thus an oscillation occurs in the thickness direction. At this time, nearly 100% of sound waves are reflected by the acoustic mirror layer AS, so that the oscillation is concentrated in an area  defined by the piezoelectric body 2, the semiconductor layers 1 and 3, the upper electrode E2, and the lower electrode E1.
The description is directed next to the principle of an operation of allowing this acoustic thin film resonator to have a variable frequency.
When a voltage is applied to the upper electrode E2 and the lower electrode E1, a diffusion voltage Vd that is generated by the diffusion of electric charge is amplified or attenuated. This causes the thickness of an active layer obtained by the application of an external voltage to be varied. With respect to longitudinal waves, the active layer behaves differently from the semiconductor layers, so that the resonant frequency of the acoustic thin film resonator varies with the thickness of the active layer.
Next, with reference to FIGS. 18, 19A and 19B, the following describes a conventional acoustic thin film resonator described in Patent Document 2.
FIG. 18 shows a cross-sectional structure of an acoustic thin film resonator. This acoustic thin film resonator has a structure in which a piezoelectric body 13 is sandwiched between an electrode 10 and an electrode 11, and a dielectric layer 14 whose dielectric constant varies depending on an applied direct current voltage and an electrode 12 further are formed below the electrode 11. The dielectric layer 14 and the electrode 12 form a variable capacitance CT. For use, this acoustic thin film resonator is mounted on a substrate 17 in which a cavity 16 is formed. In order to provide the cavity 16, for example, using a microfabrication method, the substrate 17 is etched partially from its rear surface so that a thin plate portion 15 is formed. An electric field is applied in a thickness direction of the piezoelectric body 13 by the electrode 10 and the electrode 11, and thus an oscillation occurs in the thickness direction.
FIGS. 19A and 19B show equivalent circuits of this acoustic thin film resonator. A thin film bulk acoustic wave resonator element 18 can be represented by an equivalent circuit that is constituted of a circuit in which a capacitance C1, an inductor L1, and a resistance R1 are connected in series,  and a capacitance C0 that is connected in parallel with this circuit. A variable capacitance element 19 can be represented by an equivalent circuit that is constituted of a variable capacitance CT. Depending on how a connection is made, the variable capacitance element 19 and the thin film bulk acoustic wave resonator element 18 may have a configuration shown in FIG. 19A in which they are connected in series, or a configuration shown in FIG. 19B in which they are connected in parallel.
A direct current voltage is applied between the electrode 12 and the electrode 11 shown in FIG. 18 so as to cause the dielectric constant of the dielectric layer 14 to vary, and thus the resonant frequency of the acoustic thin film resonator can be made variable.    Patent Document 1: JP 2004-534473 A    Patent Document 2: JP 2005-109573 A